1. Field of the Invention
The present invention relates to a board for ion generation that is capable of generating negative cluster ions with higher efficiency and an ion generating apparatus using the same.
2. Description of the Related Art
An ion generating apparatuses used for disinfection and deodorization has recently attracted attention and such apparatuses have started finding application in various types of electric apparatuses such as air purifiers, air conditioners, refrigerators and hair driers. There is known as such an ion generating apparatus using a board for ion generation having such a structure that electrodes are formed so that a dielectric body is sandwiched therebetween (see, for example, Japanese Unexamined Patent Publication JP-A 2003-36955, pages 5-6).
A conventional board for ion generation that has been used in such an ion generating apparatus will be described below using a cross-sectional view shown in FIG. 5. As shown in FIG. 5, a conventional board 101 for ion generation has generally been fabricated by a ceramic green sheet stacking method. A grid-like discharge electrode 102 comprising tungsten (W) as a main component is formed on one principal surface of a dielectric body 105 formed of alumina ceramics or the like. A flat induction electrode 103 also comprising tungsten as the main component is formed on the other principal surface of the dielectric body 105. Further, a first protective layer 106a for covering the surface of the discharge electrode 102 is provided on the one principal surface of the dielectric body 105 and a second protective layer 106b for covering the surface of the induction electrode 103 is provided on the other principal surface of the dielectric body 105.
Surface discharge directed from a surface part of the first protective layer 106a forming a zone above the discharge electrode 102 toward a discharge section 106a-A in a surface of the first protective layer 106a forming a zone above the end of the induction electrode 103 and surface discharge directed from the discharge section 106a-A in a surface forming the zone above the end of the induction electrode 103 toward the surface part of the first protective layer 106a forming a zone above the discharge electrode 102 are alternately and repeatedly generated on the surface of the first protective layer by applying an AC voltage to those discharge electrode 102 and induction electrode 103. Positive ions and negative ions, thereby, are generated in the zones where the surface discharge is generated, and positive and negative cluster ions are generated by surrounding the generated ions by H2O molecules. The positive and negative cluster ions produce active hydrogen peroxide (H2O2) and hydroxyl radicals (.OH) by chemical reactions and kill bacteria present in the air. Moreover, negative cluster ions take away hydrogen (H) from harmful substances (for example, substances producing bad odor or substances harmful for the health, such as nitrogen oxides), thereby deodorizing the air or making it harmless.
The first and second protective layers 106a, 106b are provided to prevent the discharge electrode 102 and induction electrode 103 from evaporation and volume loss due to a large amount of heat generated locally during the discharge. The first and second protective layers 106a, 106b may be formed of the same material as the dielectric body 105 or may be formed of a material, for example, a glassy material, different from that of the dielectric body 105.
The ion generating apparatus comprising such a conventional board 101 for ion generation generates positive and negative ion clusters in the air by the above-described surface discharge, kills bacteria and the like floating in the air, and renders harmful substances harmless. However, negative cluster ions are considered to also have an effect of making a person feel relaxed.
Accordingly, such ion generating apparatuses have recently found application for disinfecting, deodorizing and purifying the air in restricted spaces where the air can easily become stagnant, for example, business offices and conference rooms. Furthermore, a demand has been created for increased efficiency of ion generation and apparatuses capable of generating a larger number of negative cluster ions that produce a relaxing effect on humans. To meet this demand a technology is disclosed (see JP-A 2003-36955, pages 5-6) by which the grid spacing of the discharge electrode 102 is reduced to below the prescribed value and, at the same time, the thickness of the dielectric body 105 is increased to above the prescribed value in order to generate a lager number of negative cluster ions in the conventional board 101 for ion generation.
The ratio of increase in the number of generated negative cluster ions with respect to the number of generated positive cluster ions created by the board 101 for ion generation described in JP-A 2003-36955, pages 5-6, is known to be in the order of ten-odd percent to several tens of percent. Further, when negative ions are generated, the sine waveform of the AC voltage may be biased to the negative voltage side by using an electric circuit, as shown by a graph in FIG. 6 and the target atmosphere can be obtained by appropriately switching the mode of ion generation.
However, though positive cluster ions demonstrate a disinfecting and deodorizing effect by chemically reacting with bacteria and harmful substances and taking oxygen away from them, such cluster ions are known to be harmful to human health. In order to enhance further the relaxing effect of negative cluster ions on humans, the ratio of the number of generated negative cluster ions to the number of generated positive cluster ions has to be further increased. The inventors attempted to increase the number of generated negative ions with respect to that of positive ions by studying the shape and arrangement of electrodes, magnitude of applied voltage, material of dielectric body and the like, but it has been found that the increase ratio was at maximum about several tens of percent, and that further improvements will be needed to increase the relaxing effect.
Further, biasing the sine waveform to the further negative voltage side in order to generate negative ions has resulted in such drawbacks that the maximum negative voltage value increased and the amount of generated harmful ozone (O3) increased accordingly.